Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...

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138 3. Calibration of Terrestrial Laser Scannerthe derived horizontal distances to the computed spheres and the nominal horizontal distances providedby the interferometer for the 'free' adjustment and for the 'fix' adjustmentand for both diameters of 12 cmand 15 cm. The point cloud, from which the spheres were calculated, were acquired by the scanning mode'superhigh'. The figure shows that the results obtained by 'fix' adjustmentare much better than these ob¬tained by 'free' adjustment. It can be assumed that the results of the 'free' adjustment can be improved byapplying an additive constant because all distances are 3 mm to 4 mm to short8. The deviations increase sig¬nificantly for the 'free' adjustment from 10 m and thus agree with the results of the diameters comparison.The results based on the scanning modes 'high' and 'middle' confirm the findings. Consideringadjustment, the range from which the deviations grow strongly decreases from 10 m upthe 'free'to several meters.100000800001Ifpoints on sphere• 12cmhyperbola1 • 15cmhyperbola(AC'o 60000Q.M—oI 400003C2000011\\\\\\\ XN»\^^r__"0 2 4 6 8 10 12 14 16 18 20range [m]Figure 3.17: Development of the number ofpoints defining a sphere acquired by scanning at different ranges. Sphereswith diameters of 12 cm and 15 cm were scanned by the scanning mode 'superhigh'. The curves define a hyperbola.The spheres to be modeled are based on a point cloud which vary in the quantity of points. The quantityof the laser points decreases with the range and with the scanning modes from 'superhigh' to 'preview'. InFigure 3.17 the number of points belonging to the sphereswith 12 cm and 15 cm are shown and include thecorresponding adjustment hyperbolae for the scanning mode 'superhigh'. The adjustment hyperbolae arebased on the mathematical formula [Bronstein and Semendjajew, 1999]:f(x)=a-xb. (3.5)It can be seen that the number of points decreases rapidly according to the anglezontal and the vertical encoder. The parameters of the adjustment hyperbolaTherefore, the power b of both hyperbolae fit each other, which means the gradientsand the hyperbola decreases similarly.increment of the hori¬are summarized in Table 3.2.of the lines are identicalThe scale factor a states for the number of points accordingto therange of 1 m, which have to be different for the spheres because of the different diameters. Considering theother scanning modes, 'high' and 'middle', the number of points is lowered by a factor of four and sixteen,respectively. This reduction in the number of points is based on a sub-sampling, which means only eachnth pixel in the horizontal and vertical directions is stored depending on the highest angle resolution. Forthe scanning mode 'high', the sub-sampling is n 2 and for the scanning modes 'middle' and 'preview',=the sub-sampling is n 4 and = n 16, respectively. Thus, the number of points hitting = a spheremaximum distance for scanning a spherecan be estimated.and the8Since the spheres to be scanned are not only aligned in the x-direction or in the y-direction with respect to the local scanner systemthe resulting Cartesian coordinates of the spheres based on the azimuth and the horizontal distance corrected by the additive constanthas to be calculated. This procedure can be applied also on slope distances in 3D-space.
3.2 Distance Measurement System 39Table 3.2:Parameters of the hyperbola showing the development of the number of points belonging to the sphereaccording to the scanning mode 'superhigh'.SphereDiameter [m] Scale Factor a Power b12 cm 60,320 -1.59115 cm 90,590 -1.587Quantity is not the only aspect of importance. The quality also has a strong influence since the qualitydefines the geometry of the objects. The following Figure 3.18 shall demonstrate the importance of both thequantity and the quality of the point cloud.Depending on the handling of the point cloud, the appropriate scanning mode has to be chosen carefully.For documentation purposes the scanning modes 'middle' and possibly 'preview' provide the necessaryand desired information. In addition, if a high degree of accuracy and precision is of importance, then thescanning modes 'high' and 'superhigh' have to be used. The range for scanning objects, e.g. spheres, hasto be limited according to the accuracy. The knowledge of the parameters defining the geometry of objectsimprove the accuracy of distances significantly. Based on the desired resolution of the pointcloud and themeanrange to the object, the most appropriate scanning mode should be selected. However, more pointsand a denser point cloud do not mean a better accuracy. This is especially the case if the point spacing issmaller than the noise of the point cloud. Furthermore, other parameters influence the quality of the pointcloud considerably (cf. Section 3.4 and Section 3.5).Figure 3.18:scanning mode 'superhigh'.Points on a sphere with a diameter of lb cm in a range of 2m (left) and 19 m (right) acquired by the3.2.3 Long-Term StabilityThe distances provided by the laser scanner can vary over a long time since internal heatingeffects leadto changes of the modulation frequency, cf. Section 3.2.4, or other internal and/or external influences, e.g.atmospheric influences, cause variations in the distance.systemcan be assessed.Thus, the stability of the phase measurementFor the investigation of the long-term stability, the laser scanner and targets with different reflectivity valuesattached on the theodolite DKM1 were installed on two observation pillars.The distance between the twopillars was approximately 3 m. Three targets with reflectivity values of 90 %, 60 % and 20 % were used. Theobservation period covered 3 h bya measurement interval between two distance measurements of 1 min.
Page 1: DISS. ETH NO. 17036Calibration of a
Page 5 and 6: ZusammenfassungSeit einigen Jahren
Page 7 and 8: ContentsAbstractiiiZusammenfassungv
Page 9 and 10: Contentsix6.1.3 Results Ill6.2 Stat
Page 11 and 12: 1Introduction1.1 Terrestrial Laser
Page 13: 1.2 Motivation 3Precision [m10~1 "1
Page 16 and 17: 6 1. Introduction
Page 18 and 19: 8 2. Components of Terrestrial Lase
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Page 34 and 35: 24 3. Calibration of Terrestrial La
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Page 46 and 47: !36 3. Calibration of Terrestrial L
Page 68 and 69: Inormal axis,intersectionpoint axis
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Page 84 and 85: 74 4. Static Laser Scanning4.1.2 Mi
Page 86 and 87: 76 4. Static Laser ScanningOptical
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propagationresults series Taylorter
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cf.aroundframe regardingsetupusinga
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112 6. Applications of Terrestrial
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acquisitiondescribingFinally, thatM
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132 7. Summaryrange, achievable by
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134 7. Summary
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136 A. Imaging System of Imager 500
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138 B. Technical Data of Imager 500
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u140 C. Adjustment of SphereA =dfi,
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142 C. Adjustment of Sphere
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144 D. Electronic Circuit for Deter
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aMulti-SensorPlatform,PhDGeodesy Sw
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Accuracy-AeinTPS1200 AG, -Version 1
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-GenauigkeitsbetrachtungenInvestiga
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152 BibliographyWunderlich, T. A. [
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List of Figures3.22 Residuals of th
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156 List of Figures
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158 List of Tables
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200220072002Curriculum VitaePersona
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